The invention relates to a method for operating an internal combustion engine especially of a motor vehicle wherein fuel is injected into a combustion chamber in a stratified operation during a compression phase and wherein an injection duration is determined in the stratified operation in dependence upon the pressure difference between a combustion chamber pressure, which is present in the combustion chamber, and a fuel pressure operating on the fuel. The invention also relates to a corresponding internal combustion engine as well as a corresponding control apparatus for an internal combustion engine of this kind.
A method of the above kind and an internal combustion engine of the above kind as well as a control apparatus of the above type are known from so-called gasoline direct injections, for example, from German patent publication 196 45 715. There, fuel is injected into the combustion chamber of the engine in a homogeneous operation during the induction phase or in a stratified operation during the compression phase. The homogeneous operation is preferably provided for the full-load operation of the engine; whereas, the stratified operation is suitable for idle and part-load operation.
The fuel mass, which is to be injected into the combustion chamber, is computed by the control apparatus in dependence upon the load, which is applied to the engine, and on other operating variables of the engine. This fuel mass, which is to be injected, is converted into a base injection duration, which, under steady-state conditions, would correspond to the fuel mass to be injected. However, one cannot proceed from the steady-state condition because of the changing pressure relationships for an open injection valve. In a further computation step, the base injection duration is therefore converted into a corrected injection duration in dependence upon the pressure difference between the combustion chamber pressure and the fuel pressure. The injection valve of the engine is then driven with this corrected injection duration.
This procedure in the determination of the injection duration requires high complexity of the control apparatus with respect to computation and is furthermore not physically accurate.
It is an object of the invention to provide a method for operating an internal combustion engine with which an injection duration as accurate as possible can be determined with the least possible complexity with respect to computation.
The method of the invention is for operating an internal combustion engine including an engine of a motor vehicle wherein fuel is supplied to the engine and a combustion chamber pressure (pBr) develops in the combustion chamber of the engine during operation thereof. The method includes the steps of: determining a mean fuel pressure (pm) acting on the fuel supplied to the engine; injecting fuel into the combustion chamber in a stratified mode of operation during a compression phase; and, determining an injection duration (ti) in the stratified mode of operation in dependence upon the pressure difference (deltap) between the fuel pressure (pm) and the combustion chamber pressure (pBr).
With the determination of the mean fuel pressure, it is no longer necessary to correct the computed injection duration in additional computation steps. Monitoring the pressure difference between the fuel pressure and the combustion chamber pressure and the complexity of computation associated therewith are no longer necessary. In total, it is possible with the invention to carry out all computations for the determination of the injection duration on the basis of the mean fuel pressure. This represents a significant simplification of the computations and therefore a significant savings in computation time.
All essential influence quantities can be adequately considered simultaneously in the determination of the mean fuel pressure in accordance with the invention and for the use thereof for the further control (open loop and/or closed loop) of the is engine so that, with the invention, a high accuracy is achieved with the computation of the injection duration.
According to another embodiment of the invention, the mean fuel pressure is measured with the aid of a pressure sensor assigned to a fuel storage. A pressure sensor of this kind is usually already available so that no additional complexity with respect to components is caused hereby.
In another advantageous embodiment of the invention, the fuel pressure, which acts on the fuel, is controlled (open loop and/or closed loop) to the mean fuel pressure. In this way, it is ensured that the mean fuel pressure (which is used in accordance with the invention in the computation of the injection duration) remains essentially constant. In this way, the accuracy of the computation of the injection duration is significantly increased.
It is especially advantageous when the fuel pressure is controlled (open loop and/or closed loop) in such a manner that the mean fuel pressure is available at the center of the injection. In this way, the accuracy of the invention is improved.
In another advantageous embodiment of the invention, the combustion chamber pressure is determined on the basis of a model. This can preferably be carried out on the basis of a polytropic compression. In this context, it can be purposeful when an intake manifold pressure is determined especially with the aid of a pressure sensor mounted in the intake manifold. Alternatively, the intake manifold pressure can also be determined from the operating variables of the engine with the aid of a corresponding model.
In an advantageous embodiment of the invention, the injection duration is determined in dependence upon the mean fuel pressure, the combustion chamber pressure and a fuel mass which is to be injected. The above defines a mode of computation which has a very low complexity with respect to computation and can thereby be carried out especially rapidly by the control apparatus. It is therefore possible in accordance with the invention to convert the fuel mass, which is to be injected, into the corresponding injection duration especially rapidly and nonetheless with great accuracy.
The realization of the method of the invention in the form of a control element is of special significance. This control element is for a control apparatus of an engine, especially of a motor vehicle. A program is stored on the control element which can be run on a computing apparatus (especially on a microprocessor) and is suitable for carrying out the method of the invention. In this case, the invention is realized by a program stored on the control element so that this control element, which is provided with the program, defines the invention in the same manner as the method for which the program is suitable for carrying out. As a control element, especially an electric storage medium can be applied such as a read-only-memory or a flash memory.